From: jobst.brandt@stanfordalumni.org
Subject: Re: Big frames - big head angles, why?
Newsgroups: rec.bicycles.tech,uk.rec.cycling
Message-ID: <ovq_7.7107$TI3.54959@typhoon.sonic.net>
Date: Mon, 07 Jan 2002 23:50:44 GMT

Thomas Harrigan wrote:

> It's pretty common to see frames around 60cm with head-tube angles
> of 74degrees. Small frames tend to have slacker head angles, and
> although fork-offset can vary, it often doesn't, so I'm guessing
> smaller frames have "slower steering" on the whole. As someone who
> prefers a bike to feel stable rather than twitchy I find this
> tendancy in frame design a bit annoying,

The reason for the low head angle is to minimize foot overlap when the
cranks are horizontal.  The more there is, the longer the front of the
rider's foot will overlap the front wheel.  Taller bicycles also have
longer top tubes and more the front wheel far enough ahead to have no
overlap.  The 73-75deg head angle is derived from road shock angle
that arises from the statistical distribution of significant bumps, so
that these loads are taken axially.  With 73 deg angle, a 0.6" bump
(a board or pavement step) is taken axially in the fork.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Big frames - big head angles, why?
Newsgroups: rec.bicycles.tech
Message-ID: <XUL_7.7460$TI3.58770@typhoon.sonic.net>
Date: Wed, 09 Jan 2002 00:11:35 GMT

Harry Phinney writes:

>> The 73-75deg head angle is derived from road shock angle that
>> arises from the statistical distribution of significant bumps, so
>> that these loads are taken axially.  With 73 deg angle, a 0.6" bump
>> (a board or pavement step) is taken axially in the fork.

> That's interesting, and certainly fits with the trend of steeper
> head angles gaining popularity as the roads have improved. So why
> have road-going motorcycles stuck around 65 degree steering head
> angles (i.e. 25 degree rake angle)?

Their main load is from braking rather than small road irregularities
and I believe they are designed to allow telescopic forks to do their
diving without a large bending moment in the guides.  I know that my
old M/C girder forks were steeper.  In any case, the M/C has different
problems than those of a bicycle.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Big frames - big head angles, why?
Newsgroups: rec.bicycles.tech,uk.rec.cycling
Message-ID: <5DL%7.8497$TI3.68640@typhoon.sonic.net>
Date: Sat, 12 Jan 2002 00:41:37 GMT

Mark Hickey writes:

> There may have been other factors.  Gary Helfrich (godfather of
> Merlin fame) once built three or four bikes - identical except for
> head tube angle.  IIRC, the bikes were 72, 73 and 74 degrees.  He
> set 'em up identically and dared people to identify which was which.

I think that was because these riders did not know what test reveals
how much trail a bicycle has.  Trail becomes apparent mainly when
standing and leaning the bicycle from side to side on a hill or
sprint.  I don't believe it is detectable any other way, so just
riding around th block won't do.  My experience with trail is that
20mm definitely makes a noticeable difference.  That would be about
two degrees using the fork that was correct for one of two angles.

> Apparently, no one really could.  He also noticed that if he
> underinflated any of the front tires by 5psi, that bike would almost
> always be identified as the 72 degree bike.

From this it seems apparent that they did not perform the definitive
test of riding standing.

> Wish I could find the original text of the test - anyone have a copy
> from the good ol' days of the wreck.bicycles group?

From what you say, it may not be worth finding.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Big frames - big head angles, why?
Newsgroups: rec.bicycles.tech
Message-ID: <cS008.8697$TI3.70562@typhoon.sonic.net>
Date: Sat, 12 Jan 2002 20:18:16 GMT

Jon Isaacs writes:

>> I think that was because these riders did not know what test
>> reveals how much trail a bicycle has.  Trail becomes apparent
>> mainly when standing and leaning the bicycle from side to side on a
>> hill or sprint.  I don't believe it is detectable any other way, so
>> just riding around th block won't do.

>> My experience with trail is that 20mm definitely makes a noticeable
>> difference.  That would be about two degrees using the fork that
>> was correct for one of two angles.

> I have noticed when riding different bikes there is a significant
> difference in climbing while standing.  I have never done a careful
> study but I have guessed that it was related to the steering
> geometry, including the trail, rake, bar width and stem length.

> Maybe you could take a few moments and discuss your finding on how
> changing the trail affects the handling while "standing."

Too much trail makes the bicycle steer from side to side when standing
but gives better straight ahead stability when seated (how much is
enough).  Most riders, who ride aggressively, will not put up with
steering torque that interferes with straight riding when standing.

> In vague terms, my experience suggests that I prefer bikes with
> "racing" geometries such as my Merckx or Paramount whereas I find
> that more touring oriented bikes such as my Specialized Sequoia or a
> Trek 420 I had for a while just seem to be "floppy" when standing
> and pushing hard.  At times with these bikes the front wheel seems
> to want to tuck in.

Well, it's just that brand that gave me the experience.  AN old orange
Merckx owned by the proprietor of Avocet has a mismatched fork for the
rake angle and steers all over the place when first standing.  The
effect can be countered but it takes effort.  There is no indication
of this when seated, be that pedaling, braking, or cornering.

This is not only a racing frame design parameter.  Any bicycle should
have its offset adjusted to match its rake for neutral standing.  It
can be done.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Big frames - big head angles, why?
Newsgroups: rec.bicycles.tech
Message-ID: <CdG08.9336$TI3.78163@typhoon.sonic.net>
Date: Mon, 14 Jan 2002 19:22:10 GMT

Tom Harrigan writes:

>> Too much trail makes the bicycle steer from side to side when
>> standing but gives better straight ahead stability when seated (how
>> much is enough).  Most riders, who ride aggressively, will not put
>> up with steering torque that interferes with straight riding when
>> standing.

> Could you attach some numbers to what you consider too much (and too
> little)trail?

No.  I don't know more about it than I mentioned.  All the bicycles I
have ridden with the exception of that one Eddy Merckx rode, what I
take to be, neutral, whereas most riders who rode the Merckx found it
inconveniently interfering when standing.  It was especially so
because there was no hint of it until one stood for a sprint.

>>> In vague terms, my experience suggests that I prefer bikes with
>>> "racing" geometries such as my Merckx or Paramount whereas I find
>>> that more touring oriented bikes such as my Specialized Sequoia or
>>> a Trek 420 I had for a while just seem to be "floppy" when
>>> standing and pushing hard.  At times with these bikes the front
>>> wheel seems to want to tuck in.

>> Well, it's just that brand that gave me the experience.  An old
>> orange Merckx owned by the proprietor of Avocet has a mismatched
>> fork for the rake angle and steers all over the place when first
>> standing.  The effect can be countered but it takes effort.  There
>> is no indication of this when seated, be that pedaling, braking, or
>> cornering.

> I experience this whenever I have neglected riding my fixed wheel
> bike (73v74degrees).  It has a greater tendency to "drift" to the
> side when I stand.  Within a few minutes I don't notice it.  This
> makes me think I might cope happily with a 72degree frame, should I
> convince myself of any possible benefits.  It also seems easier to
> ride no-hands?

I don't know but assume that low trail would diminish straight ahead
stability, although I have watched many track riders cruise around the
banking no-hands.

>> This is not only a racing frame design parameter.  Any bicycle should
>> have its offset adjusted to match its rake for neutral standing.  It
>> can be done.

> I hope you don't mind me asking you a rather specific question: Given
> identical bikes, apart from a difference in head-angle one 72 degrees,
> the other 74degrees, which one in your opinion would give hands-on
> shimmy descending in a straight line at the lowest speed?

I think in that case the stem length and handlebar weight make the
biggest difference.  I haven't done a test but I suspect that a zero
reach stem with a minimal (vestigial) handlebar to facilitate getting
started would not shimmy coasting downhill no-hands.  No-hands
coasting is my usual test for shimmy.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Big frames - big head angles, why?
Newsgroups: rec.bicycles.tech
Message-ID: <AXH08.9365$TI3.78903@typhoon.sonic.net>
Date: Mon, 14 Jan 2002 21:19:28 GMT

Henrik Muenster writes:

>> I don't know but assume that low trail would diminish straight
>> ahead stability, although I have watched many track riders cruise
>> around the banking no-hands.

> The normal perception is, I guess, that track bikes are made for
> quick handling and less stability, but I read somewhere, that track
> bikes on the contrary are made to be very stable, since they don't have
> to go around corners.  Even though track bikes often have steep head
> angles leading to less trail, their forks often seem to have very little
> rake leading to more trail.  I don't know, if those outweigh each other,
> or if track bikes have more or less trail than normal road bikes.

The trail remains fairly constant with steeper rake and smaller offset.
The are interdependent.

> In this discussion, no one has mentioned the influence of the
> bottom bracket height, when going uphill standing. Bikes with higher
> bottom brackets tend to resist being tilted to the side, but strangely
> enough one (I at least) very quickly adapts to it, so a bike that felt
> strange and stiff when standing up, after only a short ride feels
> normal. This is called the stiltstep factor:
> <http://www.rivendellbicycles.com/reader_articles/stiltstep.htm>.
> Perhaps some confuse it with the tendency to steer to the side of bikes
> with lots of trail.

It is not counterforce of lateral pedaling torque but the self
steering that is involved in the pairing of rake angle and offset.  I
don't see how BB height can have a perceptible effect on tilt force,
the differences being small and the geometry of the contention doesn't
make sense.  Tilt torque is dependent on lateral spacing of the
pedals, not BB height and in that respect, there should be a large
difference in left and right, the right crank being as much as 10mm
farther out from the frame centerline.

The Rivendell item is much conjecture and little concrete statements
on where these forces arise.  As I said, BB heights vary little and
should tilt more easily if anything from geometric considerations.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: a perplexing frame design question
Newsgroups: rec.bicycles.tech
Message-ID: <LFqWa.7322$dk4.325489@typhoon.sonic.net>
Date: Fri, 01 Aug 2003 09:46:19 GMT

Greg <gregoryxg@juno.com> writes:

> Does anybody out there know what is the historical origin of the 72
> to 74 degree seat tube angle used on most modern road (not tri)
> bikes? I believe I understand the reason for the head tube angle and
> how it has gotten a bit steeper (now towards 74-75 deg.) over the
> years due to the general improvement in roads which has reduced the
> need for highly raked forks which has in turn reduced the need for a
> lot of 'fork trail' which has reduced the need for highly slack head
> tube angles (but correct me if I am wrong in any of this).

The angle is designed to optimize axial fork loading from road shock.
A three dimensional probability curve will show that with a common
road roughness there is an optimum angle to prevent the fork from
being loaded in bending both in severity and load magnitude.  In olden
days roads were unpaved and rougher, requiring a lower angle of
attack for this condition.

You can test this by riding no hands on some average roads with the
head bearing adjusted loosely enough to make clunking noises on bumps.
I think you'll be surprised how small an amplitude of road roughness
will cause reverse loads (clunks) in the head bearing.  In addition,
frame builders can tell you which way fork blades break in fatigue (at
the fork crown).  I think you'll find that with 72 degrees they break
forward more often, and at 75 degrees, rearward.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA